Round in quiz, entertaining student (5)
I believe the answer is:
plump
'round' is the definition.
(both can mean fat or chubby)
'quiz entertaining student' is the wordplay.
'quiz' becomes 'pump' (both can mean to interrogate).
'entertaining' is an insertion indicator (inviting in, as for dinner).
'student' becomes 'L' (learner, as in L-plates for learner drivers).
'pump' placed around 'l' is 'PLUMP'.
'in' is the link.
(Other definitions for plump that I've seen before include "Having a full rounded shape" , "Rather fat" , "Rotund" , "stout" , "Opt - round" .)